1. Field of the Invention
The present invention relates generally to photolithographic illumination systems.
2. Background Art
Photolithography (also called microlithography) is a semiconductor device fabrication technology. Photolithography uses ultraviolet or visible light to generate fine patterns in a semiconductor device design. Many types of semiconductor devices, such as diodes, transistors, and integrated circuits, can be fabricated using photolithographic techniques. Exposure systems or tools are used to implement photolithographic techniques, such as etching, in semiconductor fabrication. An exposure system typically includes an illumination system, a reticle (also called a mask) containing a circuit pattern, a projection system, and a wafer alignment stage for aligning a photosensitive resist covered semiconductor wafer. The illumination system illuminates a region of the reticle with a preferably rectangular slot illumination field. The projection system projects an image of the illuminated region of the reticle circuit pattern onto the wafer.
As semiconductor device manufacturing technology advances, there are ever increasing demands on each component of the photolithography system used to manufacture the semiconductor device. This includes the illumination system used to illuminate the reticle. For example, there is a need to illuminate the reticle with an illumination field having uniform irradiance. In step-and-scan photolithography, there is also a need to continuously vary a size of the illumination field in a direction perpendicular to a wafer scan direction, so that the size of the illumination field can be tailored to different applications. One factor often limiting wafer processing throughput is the amount of energy available from the illumination system. As a result, there is a need to vary the size of the illumination field without a loss of energy.
As the size of the illumination field is varied, it is important to preserve the angular distribution and characteristics of the illumination field at the reticle. To achieve this goal, the illumination system must maintain telecentric illumination at a substantially fixed numerical aperture at the reticle as the size of the illumination field is varied. Some illumination systems include a scattering optical element, such as an array, positioned before the reticle. The scattering optical element produces a desired angular light distribution that is subsequently imaged or relayed to the reticle. In such an illumination system, there is a need to maintain telecentric illumination at a substantially fixed numerical aperture at the scattering optical element, and correspondingly, at the reticle as the size of the illumination field is varied.
A standard zoom lens can vary the size of the illumination field. However, in the standard zoom lens, image magnification, and correspondingly the size of the illumination field, is inversely proportional to angular magnification. Thus, a standard zoom lens that increases the size of an image by a factor M, disadvantageously decreases the numerical aperture by a factor I/M, and fails to preserve the angular distribution of the illumination field.
Therefore, there is a need to vary the size of the illumination field (that is, magnify the illumination field) without a loss of energy, and to maintain telecentric illumination at the numerical aperture as the size of the illumination field is varied.
The present invention generally relates to illumination systems in photolithography. More specifically, the present invention relates to systems and methods for varying a size of an illumination field at a reticle in an optical system.
In one embodiment of the present invention, an illumination system, according to the present invention, includes an illumination source, a first diffractive array, a second diffractive array, and a condenser system placed in an optical path between the first diffractive array and the second diffractive array. The first diffractive array, also referred to as a field space array, is a double diffractive array. A light passing through the first diffractive array has a specific numerical aperture. The numerical aperture determines the size and/or shape of the illumination field at the reticle. In one embodiment, the first diffractive array is a diffractive grid capable of passing through light of different order of magnitude and refracting it out at various angles. In another embodiment, the first diffractive array includes a plurality of microlenses capable of refracting light at various angles. The second diffractive array, also referred to as a pupil array, is a double diffractive array similar in structure to the first diffractive array. The second diffractive array is able to expand and/or reduce the size of the illumination field formed at the reticle by a light passing through the second diffractive array.
According to a further feature, the condenser system includes a plurality of cylindrical and/or cross-cylindrical lenses having powers in scanning and/or cross-scanning directions. The condenser system includes a plurality of stationary lenses and a plurality of movable lenses. The plurality of stationary lenses includes an input lens and an output lens. The plurality of movable lenses includes a number of lenses capable of translation between the input and the output lenses. By translating movable lenses between the input and the output lenses, the condenser system expands and/or reduces the magnitude of a light passing through the condenser system and, hence, the size of the illumination field formed by the light at the reticle. In one embodiment, the condenser system has four lenses, including two stationary lenses (input and output lenses) and two movable lenses. In another embodiment, the condenser system has five lenses, including two stationary lenses (input and output lenses) and three movable lenses. Yet, other embodiments can include different numbers of lenses.
In operation, light from the illumination source is incident upon the beam conditioner. The beam conditioner conditions the light and directs it towards the first diffractive array. The first diffractive array processes the conditioned light. The conditioned light after passing through the first diffractive array has a specific numerical aperture. The conditioned light passing through first diffractive array can have a different size and/or shape numerical aperture. Light from the first diffractive array is incident upon the condenser system. The condenser system condenses the light directed from the first diffractive array. The condensed light forms a condenser system illumination field before passing through the second diffractive array. As used herein, the term condensed light means light having expanded and/or reduced magnitude. The second diffractive array processes the condensed light. The condensed light, after passing through the second diffractive array, forms an illumination field at the reticle. The properties of the illumination field are determined by the size and/or shape of the numerical aperture and by the magnification and/or reduction coefficients of the optical elements in the condenser system.
Further features and advantages of the present invention as well as the structure and operation of various embodiments of the present invention are described in detail below with reference to the accompanying drawings.